1. Field of the Invention
The present invention relates to a fuel injection system having a plurality of injectors, and particularly to a fuel injection system that executes an overlap injection operation, in which one of a plurality of injectors is energized first to inject fuel, and a next one of the injectors is energized after starting of the energization of the one of the injectors while the one of the injectors is still kept energized.
2. Description of Related Art
In one previously proposed fuel injection system, a large electric energy (high voltage) is stored in a charge capacitor. At the time of operating an injector, the electric energy, which has been stored in the charge capacitor, and electric energy, which is supplied from a constant-current circuit, are applied to a solenoid valve of the injector to improve response of the solenoid valve and thereby to improve the response of the injector (see, for example, Japanese Unexamined Patent Publication No. H07-71639).
It has been demanded to perform multiple injections (e.g., pilot injection m1, pre-injection m2, main injection m3 and after-injection m4, which are similar to those depicted in FIG. 2) per compression and expansion cycle of each cylinder to limit engine vibration and engine noise, to purify exhaust gas and to achieve high engine power and low fuel consumption at good balance.
Also, for the purpose of purifying the exhaust gas, it has been also demanded to perform one or more fuel injections (post-injection p1, which is similar to one depicted in FIG. 2) after combustion of the fuel in each corresponding cylinder.
With reference to FIG. 2, one of the multiple injections (e.g., the pilot injection m1) of one of the cylinders and the post injection p1 of another one of the cylinders may overlap with one another to perform an overlap injection operation.
In the overlap injection operation, as discussed above, one of the injectors (hereinafter referred to as an injector A or a former one of the overlapping injectors) is energized first to inject fuel, and a next one of the injectors (hereinafter, referred to as an injector B or a latter one of the overlapping injectors) is energized after starting of the energization of the one of injectors while the one of the injectors is still kept energized. As shown in FIG. 1A, it is conceivable to provide a plurality of charge capacitors in a charge circuit of an injector drive circuit. One of the charge capacitors supplies electric energy to the injector A, and another one of the charge capacitors supplies electric energy to the injector B. In this way, the high voltage is supplied to the injections A, B from the different charge capacitors, respectively.
A ground (GND) of the injector drive circuit is common to all of the injectors. Thus, the electric potential of the ground is increased right after application of the electric energy to the injector A. Thus, when the electric energy is supplied to the injector B right after the application of the electric energy to the injector A, the discharge current of the charge capacitor at the time of energizing the injector B is reduced. More specifically, the electric current supplied to the injector B is reduced.
When the discharge current of the charge capacitor is reduced at the time of energizing the injector B, the valve opening response of the injector B is reduced to reduce the accuracy of the injection amount in the injector B.
Due to the above described reason, the overlap injection operation has been avoided.
However, due to the diversification of fuel injection (e.g., the above multiple injections) and/or adaptation of the post processing system (e.g., the above post-injection), there is a strong demand for the overlap injection operation.